1. Field of the Invention
The present invention relates to a hub for a wind turbine.
2. Description of the Related Art
Current large-scale horizontal axis wind turbines have tower head weights (including the rotor, nacelle and drive train) of the order of 120 to 200 metric tonnes. There is an increasing trend for larger diameter turbines and the weight of the tower head is increasing approximately as the cube of the diameter of the turbine. The rotor itself (made up of the hub and blades) accounts for roughly 30% of the tower head weight. Approximately 60% of this is attributed to the blades while 40% is attributable to the hub.
U.S. Pat. No. 4,029,434 discloses the blade mounting for a windmill. The root of the blade extends into the hub where it is supported by a journal bearing assembly and a combined journal and thrust bearing assembly which allow the blade to rotate about its axis. The combined journal and thrust bearing assembly must be built around the root once the root is in situ. Further, the blade root bears directly against the two bearings and therefore must have a circular cross section. The mounting is suitable for a windmill in the 1970s (which would have had a rotor diameter of less than 20 m), but is not suitable for a modern day wind turbine blade, the blade length of which could be in order of magnitude greater than the blade contemplated by U.S. Pat. No. 4,029,434.
U.S. Pat. No. 4,668,109 discloses a bearing assembly for a wind turbine. The bearing is a sealed unit which has an outer cylinder which is bolted to the hub by an array of bolts. Within the cylinder is a shaft which is supported on a pair of bearings. A wind turbine blade terminates in a flange which is bolted by an array of bolts to a flange which is integral with the shaft. The bearing has an expansive pressure ring which is arranged to apply equal compressive force to the bearings so that the pressure is maintained as the bearing wears. The bearing is designed to be suitable for a small scale wind turbine. The manner in which the bearing is connected makes it unsuitable for a modern day large wind turbine. In particular, the requirement for two arrays of bolted joints, one at either end of the bearing would make the joint too heavy to be scaled up to the size required for a modern day turbine. Its use in a modern large scale wind turbine blade would only make the problems referred to below with regard to the plurality of bolts worse.
The current design of a conventional wind turbine rotor is shown in FIGS. 1 to 3. The rotor comprises a hub 1 which is a large, heavy and typically cast metallic structure. Three blades 2 are attached to the hub, one of which is shown in FIG. 1. The hub has a rotor axis 3 about which the rotor rotates and the blades are rotatably mounted so as to be rotatable about a pitch axis 4 each driven by a pitch motor (not shown). For each blade, the hub is provided with an annular pitch bearing 5 which supports the blade 2 so as to allow it to rotate about the pitch axis. The pitch bearing typically has an outer race 6 and an inner race 7 with a pair of ball sets 8 inbetween.
Current art large wind turbines use two types of general blade design, those with a structural spar bonded inside an aerodynamic shell and those with the stiffening structure within the aerodynamic shell. In both cases the main structural elements of each blade are terminated at the hub end in what is known as a root structure. This is the last piece of blade structure (typically 3-8 m in length) at the proximal end of the blade. This root structure takes all of the bending loads out of the blade and into a cylindrical shape ready for transfer to the hub via the pitch bearing.
The root end of the blade has a number of bolt attachment points 9 (typically 60 to 80) around the circumference of the root. These take the form of holes 10 into which threaded steel inserts 11 are bonded. A plurality of bolts 12 are inserted through the inner race 7 and into the inserts 11 to hold the blade 2 in place.
The current design has a number of shortcomings.
The rotor mass is significant both in terms of the load on the drive train and also the tower head mass. This has a significant effect, particularly for large turbines, on the dynamic interaction between the rotor and the tower. For off-shore installations, a large tower head mass is one of the significant problems with cost-effective deployment of the technology in this environment.
The inserts 11 are very difficult to produce with a high degree of repeatability. These are one of the most highly loaded points on the blade structure yet this relies on a number of secondary bonds where very high performance adhesive is used to bond the metallic studs to the composite root component.
In addition, the inserts are typically metallic and can cause problems due to differences in thermal expansion coefficient relative to the composite root structure, as well as difficulties in bond adhesion to the steel insert. Additionally, thicker sections of composite are needed at the root end of the blade to reduce flexural mismatch with the metallic inserts. This leads to the root end of the blade being heavy.
The pitch bearing also has to take the full flapwise (MFlap in FIG. 3) and edgewise (MEdge) bending moments of the blade. It also has to take the axial load (FAxial) caused by centrifugal and gravitational loading as well as radial flap-wise (FFlap) and edgewise (FEdge) loads. This means that the bearings are large diameter, expensive and heavy components in order to be able to cope with the large and varied forces. A number of pitch bearings have failed in use under these loads.
The large diameter required for the pitch bearing for the reasons set out above means that the root end of the blade needs to be made thicker (larger diameter) than is desirable for aerodynamic performance, thereby decreasing the efficiency of the blade.
The assembly of the blade onto the hub requires accurate torquing of a large number of bolts in order to achieve adequate fatigue resistance at the bolts and to avoid distortion of the pitch bearing. This is a time-consuming process which must be carried out with great care if problems are to be avoided.